Calcareous nannoplankton of the Upper Cretaceous (Campanian and Maastrichtian) of the southern and eastern Russian Platform
, Russian Plate
, East European Platform
, quantitative analysis
, regional schemes
, zonal schemes
, количественный анализ
, региональные шкалы
, зональные шкалы
, Русская плита
, Восточно-Европейская платформа
The undertaken research has resulted in the monographic description of the Campanian and Maastrichtian calcareous nannofossils of the South Russian Platform and in clarification of their taxonomic composition. Described are 117 species and three subspecies belonging to 51 genera, 18 families, five orders and four genera of unclear taxonomic position, as well as calcareous cysts of one family of dinoflagellates.
The stratigraphic distribution of the calcareous nannoplankton in 17 sections of the Russian Plate is studied in detail, and the sections are subdivided according to the standard zonal scale (Sissingh, 1977) modified by K. Perch-Nilsen (1985), and Burnett’s (1998) boreal scale. It is found that the calcareous nannofossils are present in rather large numbers in all studied sections of the Campanian and Maastrichtian. Species of the genera Prediscosphaera, Arkhangelskiella, Reinhardtites, Broinsonia and Eiffellithus, as well as Cribrosphaerella ehrenbergii are dominant. The complete absence or extreme rarity of the warm-water forms Ceratolithoides verbeekii, C. aculeus, representatives of the genus Uniplanarius and Cylindralithus arcuatus, which are used in the Sissingh’s scale for zonation of the Upper Campanian, should be noted.
The analysis of applicability of Perch-Nilsen and Burnett scales has shown that neither of them can be used on the Russian Platform in the studied interval in their entirety due to the absence of a number of warm-water taxa that define zonal boundaries and due to differences in stratigraphic ranges of some important forms. Therefore a regional zonal scheme for sequencing the Upper Santonian, Campanian and Maastrichtian deposits is offered. It consists of eight zones and 14 subzones: Arkhangelskiella cymbiformis (subzones ‘a’ and ‘b’), Broinsonia parca parca (subzones ‘a’, ‘b’ and ‘c’), Misceomarginatus pleniporus (subzones ‘a’, ‘b’, ‘c’ and ‘d’), Broinsonia parca constricta (subzones ‘a’, ‘b’ and ‘c’), Tranolithus orionatus, Reinhardtites levis, Prediscosphaera bukryii and Nephrolithus frequens (subzones ‘a’ and ‘b’).
The offered version of the zonal scheme is based on a combination of zones by Burnett, Sissingh and Perch-Nilsen. The zones are named after the index species, which first and last occurrences define the zonal boundaries, except for zone Prediscosphaera bukryii, which is named after the species that appears near the base of the interval. Nannoplankton zones are correlated with the benthic foraminiferan zonation (Olferiev & Alekseev, 2003) that has been chosen as an independent scale.
There are considerable differences in the position of zonal groups in different sections compared to benthic foraminferan zones; this can most probably be explained by the diachronous distribution of nannofossils, dependence of the stratigraphic ranges on the local ecology and other reasons.
Fluctuations of surface temperatures of the basin during the Campanian and Maastrichtian are studied on the basis of a ratio warm-water (Watznaueria barnesae, Lithraphidites carniolensis and L. quadratus) and cold-water (Micula decussata, M. concava, Arkhangelskiella cymbiformis, A. specillata, Reinhardtites levis, R. anthophorus, Broinsonia spp., Kamptnerius magnificus, Nephrolithus frequens, Prediscosphaera spp., Cribrosphaerella ehrenbergii, Eiffellithus turriseiffelii and E. eximius) taxa of the calcareous nannoplankton. The quantitative analysis of the nannofossils assemblages is based on two Campanian successions in the Belgorod Region (100 Butovo borehole and Belgorod section) and three Maastrichtian sections in the Saratov Region (Lokh, Klyuchi 1 and Klyuchi 2). Climatic changes have been visualised against the time scale (Fig. 67). Age datings are based on the age of the Santonian/Campanian boundary (83.5 mya) and main bioevents in the benthic foraminifera (Hardenbol et al. 1998) for the Campanian, and on the main bioevents in calcareous nannofossils for the Maastrichtian (Hardenbol et al. 1998).
There are phases corresponding to different temperature conditions, ten phases in the Campanian and four in the Maastrichtian. In general, the sea basin was cooler at the beginning of the Campanian than during the Late Santonian, when the abundance of warm-water forms rose up 40%. A significant cooling event started at the end of the Late Santonian, reaching its maximum at the end of the Early Campanian. The proportion of warm-water species constituted 20–25% in the Early Campanian, considerably decreasing to 8.2% towards the end of the Early Campanian. Their ratio somewhat increased, on average to 15–18%, in the Late Campanian, which apparently reflected a short-term warming period. A general picture of the graph and average values of warm-water species show a tendency towards cooling.
At the beginning of the Early Maastrichtian, the sea basin was quite cold and cooler than at the beginning of the Late Campanian in the Belgorod region, where the abundance of same warm-water nannofossil groups constituted on average 20%. Cold-water taxa were dominant (91.5–97%) in the second half of the Early Maastrichtian as well, whereas the abundance of warm-water species decreased to its minimum (3%). This interval (subzone CC23b and zone CC24) is approximately synchronous to the break established on the Atlantic coast of the USA in the State of New Jersey (Sugarman et al. 1995; Miller et al. 1999) where zones CC23 and CC24 are absent from many sections. The formation of this break is linked to the drop of the ocean level by 30–40 m, caused by a global cooling (Miller et al. 1999), which is confirmed by our data.
The end Late Maastrichtian warming (phases 3 and 4, interval of 67.7–65.1 mya) is not so well expressed in other regions, where it looks more short-term and encompassing only last 350–500 thousand years of the Maastrichtian age (Li & Keller, 1998; Barrera & Savin, 1999; Olsson et al. 2001). In the Saratov Volga Region, this short warming event is fixed well by the assemblage of planktonic foraminifera, with a peak at about 65.3 mya. It should be noted that in Tunisia, where the Upper Maastrichtian is characterized by a a considerable thickness, alternating three warming and three cooling events have been detected on the basis of isotope studies in the interval of 65.0–65.7 mya (Stüben et al., 2003). The worming events have been established in intervals 65.33–65.38, 65.12–65.26 и 65.00–65.04 mya. Thus, the oldest event, which begins with the gradual temperature increase 65.5 mya, coincides precisely with warming registered in the Volga Region on planktonic foraminifera.
Quantitative changes of calcareous nannoplankton fix rather reliably temperature changes of surface waters of ancient sea basins, revealed by more sensitive isotope methods. However, changes in nannofossils assemblages must be compared with those in other groups to get more reliable results.
Data from the southern part of the East European Platform confirm the global nature of the Campanian cooling trend, which is clearly expressed at middle latitudes of Northern Peri-Tethys. An overall trend towards considerable global warming at the end of the Late Maastrichtian is also confirmed. At the same time, the terminal Maastrichtian cooling revealed by Gorostidi and Lamolda (1995) in Spain has not been established, apparently due to a break at the Cretaceous / Paleogene boundary in the studied sections of the Saratov Region.
The calcareous nannoplankton can be used for a fine subdivision of Upper Cretaceous sediments of the Russian Platform and adjacent areas, and for clarification and detalisation of the Upper Cretaceous stratigraphic scheme. The results of the study have been used in the development of the regional stratigraphic scheme of the Upper Cretaceous of the Russian Platform (Olferiev & Alekseev, 2005).
During further research, it would be necessary to study the diachroneity in the distribution of nannoplankton species, using palaeomagnethic or chemostratigraphic methods as independent correlation tools. Of great interest is the investigation into latitudinal changes of calcareous nannoplankton assemblages along the profile from the Central Volga Area to the Crimea and North Caucasus. Finally, the detail study of the fine nannofossils morphology should facilitate the clarification of their taxonomy at the level of species and subspecies, for some anomalies in the stratigraphic ranges of individual forms can be explained by the composite nature of certain taxa.